This invention relates to apparatus and techniques for making nuclear magnetic resonance (NMR) measurements in boreholes, and to methods for determining magnetic characteristics of formations traversed by a borehole. Specifically, the invention relates to a side-looking NMR tool that attenuates NMR signals from the borehole while maintaining a large region of investigation within the formation.
A variety of techniques have been used in determining the presence and in estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, porosity, fluid content, and permeability of the rock formation surrounding the wellbore drilled for recovering hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) or logging-while-drilling (xe2x80x9cLWDxe2x80x9d). Measurements have also been made when tripping a drillstring out of a wellbore: this is called measurement-while-tripping (xe2x80x9cMWTxe2x80x9d).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as xe2x80x9cT1xe2x80x9d), and transverse relaxation time (generally referred to as xe2x80x9cT2xe2x80x9d) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B0 in the vicinity of the wellbore, and an oscillating field B1 in a direction perpendicular to B0. This oscillating field is usually applied in the form of short duration pulses. The purpose of the B0 field is to polarize the magnetic moments of nuclei parallel to the static field and the purpose of the B1 field is to rotate the magnetic moments by an angle xcex8 controlled by the width tp and the amplitude B1 of the oscillating pulse. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (xe2x80x9cCPMGxe2x80x9d) sequence that can be expressed as
TWxe2x88x9290xe2x88x92(txe2x88x92180xe2x88x92txe2x88x92echo)nxe2x80x83xe2x80x83(1)
where TW is a wait time, 90 is a 90 degree tipping pulse, 180 and is a 180 degree refocusing pulse.
After being tipped by 90xc2x0, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency xcfx890, given by xcfx890=xcex3 B0, where B0 is the field strength of the static magnetic field and xcex3 is the gyromagnetic ratio. At the same time, the magnetic moments return to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the xe2x80x9cspin-lattice relaxation timexe2x80x9d or T1. Inhomogeneities of the B0 field result in dephasing of the magnetic moments and to remedy this, a 180xc2x0 pulse is included in the sequence to refocus the magnetic moments. This gives a sequence of n echo signals. These echo sequences are then processed to provide information about the relaxation times.
U.S. Pat. No. 4,350,955 to Jackson et al discloses a pair of permanent magnets arranged axially within the borehole so their fields oppose, producing a region near the plane perpendicular to the axis, midway between the sources, where the radial component of the field goes through a maximum. Near the maximum, the field is homogeneous over a toroidal zone centered around the borehole. With the Jackson arrangement, the axial extent of the region of examination is quite limited. As a result of this, the device can only be operated at relatively low logging speeds: otherwise, because of the tool motion during logging, the magnitude of the static field changes significantly within a fixed region of the formation with an accompanying degradation of NMR signals.
There are three approaches that may be taken in the design of an eccentric logging tool. One approach is to have a static field defining a region of examination that is primarily on one side of the tool. A second approach is to have a RF antenna that is primarily sensitive to signals from one side of the tool. The third approach is to have both the static field and the RF antenna with directional sensitivity.
U.S. Pat. No. 5,488,342, to Hanley, discloses a variation of the Jackson device wherein a shaping magnet is positioned adjacent the space between the pair of opposed magnets with its magnetic axis transverse to the borehole axis. The arrangement in the Hanley ""342 patent has a region of uniform static field that is limited to one side of the magnet arrangement. U.S. Pat. No. 5,646,528 also to Hanley, discloses another variation of the Jackson device in which a shield of electrically conductive material is positioned adjacent to and laterally offset from the set of electrical coils whereby the magnetic field generated by the RF antenna is asymmetrically offset from the axis of the first magnets. The region of uniform static field remains a toroid, as in the Jackson device. The Hanley ""528 device may be operated eccentrically within a large borehole with a reduction in the borehole signal. Both of the Hanley devices suffer from the drawback that the axial extent of the region of examination is small, so that they cannot be operated at high logging speeds.
There are several devices in which the problem of limited axial extent of the basic Jackson configuration of permanent magnets is addressed. U.S. Pat. No. 4,717,877 to Taicher et al teaches the use of elongated cylindrical permanent magnets in which the poles are on opposite curved faces of the magnet. The static field from such a magnet is like that of a dipole centered on the geometric axis of the elongated magnets and provides a region of examination that is elongated parallel to the borehole axis. The RF coil in the Taicher device is also a dipole antenna with its center coincident with the geometric axis of the magnet, thereby providing orthogonality of the static and magnetic field over a full 360xc2x0 azimuth around the borehole.
U.S. Pat. 6,023,164 to Prammer discloses a variation of the Taicher patent in which the tool is operated eccentrically within the borehole. In the Prammer device, NMR logging probe is provided with a sleeve having a semi-circular RF shield covering one of the poles of the magnet: the shield blocks signals from one side of the probe. The probe is provided with elements that press the uncovered side of the probe to the sidewall of the borehole so that signals from the uncovered side arise primarily from the formation.
For both the Prammer and the Hanley ""528 devices, in order to get the best attenuation in the field behind the probe while maintaining sensitivity in front of the probe, the shield should be positioned as far away from the front region as possible. The effectiveness of the shield is limited by the diameter of the tool. In the absence of a shield, the Prammer and Hanley ""528 tools have a circular sensitive region, so that use of either device in an eccentric manner would result in a large signal from the borehole fluid.
U.S. Pat. No. 5,055,787 to Kleinberg et al combines the RF shield concept taught in Prammer with a shaping of the static field and with actually separating the effective center of the RF dipole antenna and the center of the magnet arrangement. Three magnets with parallel magnetization are used to produce the static field, the center magnet being opposed in polarity to the magnets on either side. The device has a region in front of the tool with a zero gradient while the region behind the tool has a large gradient. Consequently, the volume of the sensitive region in front of the tool is much larger than the sensitive region behind the tool, so that borehole signals are greatly reduced. One drawback of the Kleinberg arrangement is that the region of examination is very close to the tool. This makes it difficult to make measurement deeper into the formation, a serious drawback if there is significant invasion of the formation by borehole fluids. The Kleinberg device is also what is known as a zero gradient tool, i.e., the static field has substantially zero gradient in the region of examination. This is a disadvantage in NMR logging because many interpretation techniques for deriving petrophysical information about fluid diffusion in the formation from NMR data depend upon having a known and finite gradient.
The present invention is a side-looking NMR probe for well logging applications. It incorporates a number of design features, some of which are novel in themselves, in a novel combination that greatly improves the effectiveness of the tool. The starting point is a configuration that includes a static field similar to that of a dipole, and a dipole-like RF field substantially orthogonal to the static field, with the centers of the equivalent static and RF dipoles laterally displaced to provide a match on a side defined as the front of the tool and a mismatch on the back of the tool. The basic static field may be produced by a main magnet. The static field in the region of investigation has a field strength within predetermined limits and a substantially uniform gradient. Those versed in the art would recognize that in a so-called zero gradient logging tool, in contrast, the static field in the region of investigation has a substantially uniform field strength around a saddle point. A second, shaping magnet is used to shape the static field to conform to the RF field over a larger azimuthal sector around the tool. The RF field is also shaped to increase the effective radius in the front of the tool, giving a greater depth of penetration, and making it conform to the static field over a larger azimuthal sector. The antenna dipole is configured with as large a dipole as possible, thereby increasing its efficiency. A static shield may be used to reduce the RF field behind the tool. The static and RF dipoles are rotated 90xc2x0 relative to prior art, so that the static dipole points to the side of the tool and the RF dipole to the front of the tool. With this arrangement, eddy currents in the shield are substantially increased, increasing its effectiveness. The RF antenna includes a core made of a soft magnetic material to increase its efficiency. The shaping magnet also acts as a bucking magnet to allow use of the core material: in its absence, the static field would be shorted out due to the rotated field orientation. This greatly reduces the field in the core and hence also reduces magnetostrictive ringing. In addition, the ringing in the core is reduced by using a soft magnetic material comprising particles of powdered material small enough to be transparent to the RF magnetic field